1) Field of the Invention
The present invention relates to a method for transmitting data from a first network node (e.g., an information server) to a second network node (e.g., a client) through an interlinking network which includes a plurality of transmission equipments. The present invention also relates to a data transmission system including a plurality of transmission equipments and transmitting data from a first network node to a second network node. The present invention further relates to a transmission equipment used in a data transmission system interconnecting first and second network nodes.
2) Description of the Related Art
Recently, the bandwidths of local networks have been increased, and the bandwidths of the access networks have also been increased by the development of the technologies of XDSL (X Digital Subscriber Line), FTTH (Fiber To The Home), and the like. With the increase in the bandwidths of the local and access networks, business opportunities are increasing, and various services are becoming available. In such a situation, increase in the bandwidths of backbone or interlinking networks (e.g., use of broadband networks) is also required.
In particular, attempts are currently made to transmit data requiring wide bandwidth (e.g., sound data or image data) by using the IP (Internet Protocol) platform. For example, in some applications such as delivery of radio programs or music, the attempted data transmission has become commercially practical. In addition, low-quality image delivery services have already become commercially practical.
Currently, in order to realize the above data transmission using the IP platform, many distributed systems have been constructed by arranging a number of cache servers in the Internet. In such distributed systems, data are delivered by network elements using the IP multicast technique.
In addition, in order to increase the overall throughput of the data transmission system and realize music or images delivery services to a wide area, attempts have been made to directly connect a LAN (Local Area Network) to a broadband, backbone or interlinking network such as a SONET (or SDH) network or a WDM (Wavelength Division Multiplex) network. It is considered that the direct connection of a LAN to a SONET (or SDH) network or a WDM network will be widely used in the future.
Next, the data transmission using the IP platform and the transmission through the SONET (or SDH) network are explained below.
(i) Data Transmission Using IP Platform
FIG. 20 is a diagram illustrating an example of a conventional image delivery or cable television system which uses the Internet. The example of FIG. 20 has a typical LAN-WAN (Wide Area Network)-LAN configuration.
In FIG. 20, reference numeral 10 denotes a camera, 11 denotes an image transfer equipment, 12 denotes an image server, 13 denotes an L2 (Layer 2) switch, 14 denotes an IIP (Internet Imaging Protocol) router, 15 denotes a WAN, 16 denotes an IIP router, 17 denotes an image server, 18 denotes an L2 switch, and 19-1 to 19-3 each denote a client.
The camera 10 captures an image of an object and sound, converts the image and sound into digital data, and outputs the digital data. The image transfer equipment 11 generates an IP packet containing the digital data output from the camera 10, and transmits the IP packet through the L2 switch 13. Each of the image servers 12 and 17 temporarily stores image data, and transmits the image data in response to a request from the clients 19-1 to 19-3. Each of the L2 switches 13 and 18 performs packet switching in the data link layer. Each of the IIP routers 14 and 16 transfers image data in accordance with a protocol for exchanging image data and related information.
For example, the WAN 15 is realized by an ATM (Asynchronous Transfer Mode) network, a SONET (or SDH) network, an ISDN network, or the like, and uses the MPLS (Multiprotocol Label Switching) technique.
Each of the clients 19-1 to 19-3 receives image data and displays an image.
The image data and the sound data output from the camera 10 are delivered to the clients 19-1 to 19-3 in different manners according to the services requested by the clients 19-1 to 19-3.
In a first type of service, the image data and the sound data output from the camera 10 are temporarily stored in the image server 17. Thereafter, when one of the clients 19-1 to 19-3 makes a request for the image data and the sound data, the image data and the sound data are transmitted to the one of the clients 19-1 to 19-3 through the L2 switch 18.
In a second type of service (real-time delivery service), the image data and the sound data output from the camera 10 are first contained in an IP packet by the image transfer equipment 11, and then multicast delivered to the clients 19-1 to 19-3 by using the IP multicast technique.
However, the volume of image data transmitted in the conventional image delivery or cable television system is basically great. In particular, television broadcasting constantly requires for each channel a bandwidth of 3 to 10 Mbit/s in the case of standard definition television data and 20 Mbit/s in the case of high definition television data. In the case of sound data, a bandwidth of 80 to 130 Kbit/s is required.
Therefore, the conventional system is sufficient to deliver the sound data. However, in order to deliver the image data, it is necessary to control the bandwidth and increase the transmission rate in the entire system including the LANs and WANs. For example, in the case where 10 channels of high definition television data are delivered, a bandwidth of 200 Mbps (=20 Mbps×10 ch) is constantly required.
Nevertheless, the constant flow of the above traffic impedes transmission of other information, and is not economical.
It is well known that bottlenecks in networks are typically caused by differences in the interface speed between LANs and WANs. For example, many WANs have ISDN (64/1,544 Kbps DS1), ATM (155 Mbps), or XDSL interfaces for connection to users. In addition, in an increasing number of cases of transmission, data transmitted from the users through the ISDN, ATM, or XDSL interfaces are multiplexed by using the SONET or SDH technology and transmitted through transmission lines of OC-3 (155 Mbps), OC-12 (622 Mbps), OC-48 (2.5 Gbps), OC-192 (10 Gbps), or the like. In order to realize the above operation, for example, a POS (Packet Over Sonet) unit is installed in IP routers. On the other hand, recently, many LANs uses the Ethernet having a transmission rate of 100 Mbps or 1 Gbps. That is, the bandwidths of the interfaces between the WANs and the LANs are much smaller than those of the LANs and WANs, and the transmission is delayed by the small bandwidths of the interfaces between the WANs and the LANs.
Further, in transmission of a great amount of images and sound data, images and sound received by clients (users' PCs) may be interrupted when packets are delayed, discarded, or disordered by the influence of the reduction of the bandwidth in the WAN and the best effort transmission of IP packets.
In order to solve the above problem, conventionally, cache servers are distributed in the Internet so that information is stored by the cache servers, and the delay, discard, and disorder of the packets are absorbed.
FIG. 21 is a diagram illustrating an example of a configuration of cache servers in the Internet. In FIG. 21, the other network elements such as IP routers and hubs are not shown.
As illustrated in FIG. 21, the cache servers 30a to 30d are distributed in the WAN 30, the clients 31 and 32 are connected to the cache server 30d, the clients 33 and 34 are connected to the cache server 30c, and the client 35 is connected to the cache server 30b. 
Since the cache servers 30a to 30d are arranged near the corresponding clients 31 to 35, it is possible to avoid inefficient access to original contents in response to each connection request, improve response, and reduce traffic.
(ii) Conventional SONET or SDH System
The SONET transmission equipments are high-speed transmission equipments mainly used in backbone networks of network providers, and various data such as data of telephones, leased lines, and IP packets, for which the network providers provide service, are transmitted through the SONET transmission equipments. The SONET transmission equipments multiplex such data into a high-speed transmission signal having a transmission rate of 2.5 Gbps, 10 Gbps, 40 Gbps, or the like, and transmit the high-speed transmission signal through a single optical fiber, by using the SONET or SDH technology. In SONET or SDH networks, each path has uniform traffic, and synchronism is maintained.
FIG. 22 is a diagram for explaining the bandwidths of incoming lines and corresponding outgoing lines. In the path indicated by the double lines with arrows the example of FIG. 22, the transmission line incoming into the SONET transmission equipment 40 and the transmission line outgoing from the SONET transmission equipment 41 have an identical transmission rate 2.5 Gbps. On the other hand, in the path indicated by the single lines with arrows, the transmission line incoming into the SONET transmission equipment 43 has an transmission rate 2.5 Gbps, and the transmission line outgoing from the SONET transmission equipment 42 has an transmission rate 622 Mbps. This is impractical.
In the packet transmission of data, basically, more than one network terminal shares a transmission line (i.e., a bandwidth of a transmission line), and the synchronism is not secured. On the other hand, in the SONET networks, bandwidth allocation to each transmission line is predetermined, and the synchronism is maintained.
However, conventionally, in order to provide a broadband service in a wide area, cache servers and other network elements are required to be distributed over the wide area under various conditions. Therefore, sufficient bandwidths are not necessarily secured between a data source server and the cache servers. In this case, the amount of data output from the data source server is limited. Thus, even when the bandwidth is great in only a portion (e.g., in an interlinking network such as a SONET or SDH network) of a transmission path between the data source server and each cache server, the great bandwidth of the portion cannot be efficiently used.
Further, in a broadband service such as an image or sound delivery service, data sent to the respective cache servers are often identical. However, conventionally, the data source server is required to send the identical data to each cache server separately. FIG. 23 is a diagram illustrating an example of a configuration for transmitting data from a data source server to a plurality of cache servers in a conventional data delivery service system. In the example of FIG. 23, the contents server (data source server) 51 is required to send identical data to each of the cache servers 52 to 54, i.e., the contents server 51 is required to transmit the identical data three times. Therefore, utilization efficiency of the bandwidth of the WAN 50 is low.